Process for the purification of tar acids and product thereof



Patented Sept. 12, 1933 PATENT OFFICE PROCESS FOR THE PURIFICATION OF TAR ACIDS AND PRODUCT THEREOF Clifford Banta, Philadelphia, Pa., assigzior to The Barrett Company, New York, N. Y., a corporation of New Jersey No Drawing. Application August 28, 1929 Serial No. 389,100

11 Claims. (Cl. 20252) This invention relates to a process for thepurlfication of tar acids, particularly to a process for the reduction of the sulphur content of tar acids, and to the purified product so obtained.

Tar acids are a product of the coal tar industry and, as obtained, may contain phenol, cresols,

' xylenols and related aromatic hydroxy compounds, hereinafter referred to as tar acids. Commercial tar acids as ordinarily prepared consist for the most part of mixtures of phenol, cresols and higher homologues thereof. The tar acids are commonly recovered by washing the oil containing them with caustic soda solution and then liberating the tar acids from the resultant aqueous extract by adding sufficient inorganic acid to react with the basic radical. The crude tar acid will contain considerable water and up to 0.5% of sulphur on the average. The sulphur content may be even as much as 3% but a crude acid containing a far less quantity, e. g., as little as 0.2% of sulphur, when subjected to the ordinary distillation may still give an objectionable, foul-smelling tar acid product.

The sulphur content of distilled or refined tar acids has been a source of trouble for many years. In some cases refined .acids are rendered unmarketable on account of the strong odor of the sulphur compounds contained in them; furthermore, the presence of sulphur in tar acids increases their tendency to turn dark in color and renders them objectionable to manufacturers of synthetic resins and other phenolic products. This sulphur, in the form of its compounds, has been difficult to remove to any great extent. It is the primary object of this invention to provide a process for the reduction of the sulphur content of distilled tar acids to a point where a satisfactory commercial product will be obtained. A further object is to increase the yield of tar acids by the use of milder conditions of heating, thus avoiding decomposition. The process is applicable not only to crude tar acids as usually produced in the coal tar industry but to tar acids from other sources and processes and to the individual acids such as phenol, cresols, xylenols, and others.

In experiments conducted with the object of removing the objectionable sulphur compounds, it was discovered that the sulphur content of the distilled tar acids varied with the absolute pressure under which the distillation was conducted. Further investigation resulted in the development of a process which would satisfactorily free the tar acids from their sulphur content in a very simple manner and which comprises subjecting the tar acid to vacuum distillation. I

have found that distillation of the crude tar acids, ordinarily without any attempt at fractionation, under an absolute pressure of less than about 75 mm. of mercury and preferably not over 25 mm. of mercury will satisfactorily free the tar acids from objectionable sulphur compounds. Thelowest pressure within this range which can be maintained in commercial equipment is,-in general, advisable since it has been found that the higher the degree of vacuum attained the better is the separation of tar acids from the sulphur compounds. A very effective pressure has been found to be 15 mm. absolute or, in other terms, a vacuum of approximately 29.5 inches of mercury. The great reduction of the pressure 7 upon the tar acids in the still results in a considerable lowering of the temperature at which the various constituents of the crude acid boil. For example, at an absolute pressure of 15 mm. of mercury the boiling range of the acids is lowered about 100 to 110 C. from what it would be at atmospheric pressure, for instance the peak temperature of distillation may be lowered to 160 C. but preferably should not exceed about 125 C.

In carrying out the distillation, care must be taken in regulating the heat supplied to the liquid in order not to cause local overheating, resulting in decomposition of the sulphur compounds' in the boiling liquid and contamination of the distillate with the products of decomposition. The heat supplied for distillation should be limited to an amount required to effect distillation and should be supplied at a temperature as low as is practicable for this purpose. Under this pressure of 15 mm. of mercury absolute, the tar acids boil off at a. temperature which is found to be sufllciently below both the boiling points and the decomposition temperature of the major portion of the sulphur compounds so that the bulk of these compounds is left in the residue and the tar acids pass over almost free therefrom.

Gaseous sulphur compounds, such as hydrogen sulphide, as well as non-volatile sulphur compounds, contained in the crude tar acid are also removed by this process. Experiments have shown that, under the high vacuum maintained during the operation of this process, such gaseous compounds are rapidly evolved during the distillation and carried through the condenser to the pump without substantial condensation in the distillate. Once free of its objectionable sulphur content to the required extent, the entire tar acid distillate can be re-run in a moderate vacuum for example at an absolute pressure of from 150-250 mm. of mercury to effect a separation of any fractions desired.

As illustrative of the effect of reduction of the absolute pressure under which the distillation is effected, upon the sulphur content of the distilled tar acids, the following tabulation is set forth. A crude acid containing 0.22% sulphur was distilled at the following absolute pressures, yielding distillates with the given sulphur content:

Distilled at 760 mm 0.10% S Distilled at mm 0.030% S Distilled at 25 mm 0.020% S Distilled at 5 mm. 0.015% S It will be seen that the sulphur content of the distillate depends upon the absolute pressure prevailing during the distillation. In general, refined tar acid containing less than 0.03% of sulphur is a satisfactory productof remarkably good quality.

The following example may be given as illustrative of preferred conditions for commercial operation. A quantity of tar acids was distilled under a vacuum of 15 mm. absolute pressure which permitted the acids to boil off at a temperature of 125 C. as a maximum. Heat for the distillation was supplied by indirect contact with steam at a maximum pressure of 85 pounds per square inch, corresponding to a temperature of slightly less than 160 C. Under these conditions a distillate was obtained which, when redistilled under 760 mm. pressure, gave a dry point of 235 C. and which was of remarkable quality as to odor, color and sulphur content.

It is to be understood that the specific example given is illustrative and is not limiting. The invention embraces broadly the application of the discovery that the sulphur content of distilled tar acids decreases with decrease in the absolute pressure prevailing during the distillation of tar acids and that the sulphur content thereof is reduced by maintenance of the necessary low vacuum during the distillation operation. Therefore, while the preferred embodiment of the invention has been described above, it is intended that the invention be not limited except as set forth in the appended claims.

I claim:

1. The process of producing from crude commercial sulfur-containing tar acids, tar acids substantially free from non-gaseous sulfur compounds, which comprises distilling the sulfurcontaining tar acids under an absolute pressure less than about 75 mm. of mercury, whereby the non-gaseous sulfur content of the tar acids is retained in the residuefrom the distillation.

2. The process of producing tar acids substantially free from non-volatile sulphur compounds which comprises distilling the acids containing sulphur compounds under an absolute pressure less than 75 mm. of mercury and at a temperature of less than 160 0., whereby the decomposition of sulphur compounds is inhibited and sulfur compounds are retained in the residue from the distillation.

3. The process of producing tar acids substantially free from non-volatile sulphur compounds which comprises lowering the boiling range of said acids containing sulphur compounds about 100 C. below the normal boiling range by means which comprises distilling said acids under an absolute pressure of less than about '75 mm. of mercury thereby retaining sulfur compounds in the residue from the distillation.

5. The process of producing tar acids substantially free from non-volatile sulphur compounds from tar acids containing sulfur compounds, which comprises distilling said acids under an absolute pressure of less than 75 mm. of mercury thereby retaining sulfur compounds in the residue from the distillation, and then fractionating the distillate under a vacuum.

- 6. The process of producing tar acids substantially free from non-volatile sulphur compounds from tar acids containing sulfur compounds, which comprises distilling said acids under an absolute pressure of not more than 25 mm. of mercury.

7. The process of purifying tar acids containing non-volatile sulfur compounds, which comprises distilling said tar acids at a temperature not exceeding about 125 0., thereby distilling off tar acids as distillate and retaining sulfur compounds in the residue from the distillation.

8. Commercial tar acids, consisting for the mostpart of a mixture of phenol, cresols and higher homologues thereof, containing less than 0.03% sulphur.

9. The process of producing tar acids substantially free from non-volatile sulphur compounds from tar acids containing sulfur compounds which comprises distilling said acids under an absolute pressure of less than '75 mm. of mercury thereby retaining sulfur compounds in the residue from the distillation, and then fractionally distilling the distillate.

10. The process of producing from crude commercial tar acids containing non-volatile and gaseous sulfur compounds, tar acids substantially free from sulfur compounds, which comprises distilling the crude tar acids under an absolute pressure less than 75 mm. of mercury, whereby the decomposition of the non-volatile sulfur compounds is inhibited and the non-volatile sulfur compounds are retained in the residue from the distillation, cooling and collecting the distillate under vacuum whereby gaseous sulfur compounds are retained in the gaseous phase and tar acids are collected as substantially sulfur-free condensate.

11. The process for producing from crude commercial sulfur-containing tar acids, tar acids substantially free from non-gaseous sulfur compounds, which comprises distilling the sulfurcontaining tar acids at a temperature not exceeding about 125 C. thereby distilling off tar acids as distillate and retaining the non-gaseous sulfur compounds in the residue from the distillation.

CLIFFORD BANTA. 

